Recently, so-called electronic cameras have prevailed. In the electronic camera, an object image is formed on a semiconductor image sensing device, e.g., an image sensor via a photographing optical system. The object image is converted into an electrical signal, and image data of the obtained still image is recorded on a recording medium such as a semiconductor memory or magnetic disk.
Most electronic cameras of this type have an auto focus (AF) mechanism of controlling photographing conditions in order to automatically focus on an object image. The AF mechanism often adopts a method called contrast AF or hill-climbing AF. This is because the AF method can use an output from an image sensing device directly for AF. More specifically, in contrast AF, the contrast value (AF evaluation value) of a value output from an image sensing device is evaluated, and the electronic camera is determined to be focused at the maximum contrast value.
To the contrary, silver halide single lens reflex cameras widely employ a technique called phase difference AF in addition to contrast AF in order to achieve auto focus. Phase difference AF is advantageous in making the in-focus point coincide with the film surface by one driving because how far the in-focus point is apart from the film surface can be instantly recognized on the basis of the distance between images (phase difference) when light from the object image is received by a phase difference detection sensor having a CCD line sensor. These two AF methods are widely used in silver halide cameras.
Japanese Patent Laid-Open No. 07-043605 discloses a camera which comprises a focus detection means that detects the focus by phase difference AF and is made up of an optical system and light-receiving sensor, and a focus detection means that detects the focus by contrast AF and is made up of an optical system and light-receiving sensor. This camera uses a combination of focus detection of phase difference AF and focus detection of contrast AF, obtains a defocus amount, and focuses the lens. With this structure, the lens is focused by performing coarse adjustment by focus detection of phase difference AF and fine adjustment by focus detection of contrast AF.
Japanese Patent Laid-Open No. 09-181954 discloses a technique of moving the focus lens by the second focus control means using the position of a focus lens moved by the first focus control, detecting the maximum value of the contrast, and returning the focus lens to the position exhibiting the maximum value.
Japanese Patent Laid-Open No. 2001-281530 discloses a technique of driving the photographing lens on the basis of a phase difference detection signal, and then driving the image sensing lens at a predetermined evaluation value corresponding to the contrast of an object image from a sensed image.
For example, contrast AF focus detection has a small detectable defocus amount, and it is difficult to detect the focus when the camera is greatly out of focus. A long time is taken to scan the focus lens from infinity to the minimum object distance in order to obtain an in-focus point. Contrast AF focus detection is not suitable for a system which requires quick operation or photographing an object which moves quickly. Further, since the high-frequency component less changes at a portion apart from the in-focus point, it is difficult to know whether defocusing arises from a near-focus state or far-focus state.
Phase difference AF focus detection has a large detectable defocus amount, but a dead zone is generated in the focus detection area. Since the moving amount of the AF lens is determined in accordance with a shift of the imaging position of the image sensing device, the AF enable range (AF range) is limited depending on the image sensing device or lens system. For a large range from infinity to the minimum object distance, if the lens system is so set as to enable AF in the entire photographable range, the resolution and AF precision may decrease in terms of the balance with the device size of the image sensing device.
To solve these problems, in Japanese Patent Laid-Open No. 07-043605, the lens is focused by always performing coarse adjustment in a given distance measurement area by phase difference AF focus detection and fine adjustment by contrast AF focus detection.
In Japanese Patent Laid-Open No. 09-181954, the lens is moved to an in-focus point by the first phase difference AF, and scanned before and after the lens position by contrast AF to detect an in-focus point. The lens is scanned after it is returned to a contrast AF start position from a position at which the lens was focused once by phase difference AF. This generates a time loss.
In Japanese Patent Laid-Open No. 2001-281530, the lens of a single lens reflex type digital still camera is driven on the basis of a phase difference detection signal by phase difference AF, and then the lens is driven on the basis of an evaluation value by contrast AF. The result of the phase difference detection signal is not particularly reflected in the contrast AF lens driving control method. A long time is taken to focus the lens. Furthermore, no concrete driving sequence of contrast AF lens driving is mentioned.
According to the prior arts, the camera is focused using the two distance measurement methods in the same distance measurement area. Both phase difference AF and contrast AF are used in one distance measurement area to increase the focusing precision and focusing speed.
As described above, phase difference AF generates a dead zone in the distance measurement area, i.e., an area where the focus cannot be adjusted. In general, the area where the focus cannot be adjusted by phase difference AF can be often covered by contrast AF. In this case, focusing operation is done by only contrast AF focus adjustment. The range in which the focus can be adjusted by only contrast AF becomes relatively wide. If contrast AF is always executed in this area, a long time is-taken for focusing, and a chance to take a picture may be missed.